Materials that absorb energy to excite electrons and emit electromagnetic radiation as extra energy when the excited electrons return to the ground state exhibit wavelength conversion performance and have been used as a color (or wavelength) converting colorant in dye or pigment formulations, optical filters, and agricultural filters. In particular, organic compounds of such materials have been studied extensively because they have more easily controllable absorption and emission wavelengths than inorganic compounds. Among them, compounds that emit absorbed energy as fluorescence are called fluorescent colorants. Of the fluorescent colorants those emitting visible fluorescence are of high utility and have found application in displays, lighting equipment, such as fluorescent lamps, biological or medical markers, and so on.
Schiff base type compounds have been used in the form of their metal complexes as an optical recording material or an optical filter material as disclosed in patent documents 1 to 3 (see below). Fluorescence-emitting colorants are applicable to a high brightness, high efficiency color conversion filter. Inter alia, those emitting fluorescence in their solid state have been awaited in view of high practical utility. For example, patent documents 4 to 7 (see below) disclose compounds that emit fluorescence in their solid state. However, the colorants disclosed in the literature are not quite sufficient in terms of various performance requirements.    Patent document 1: JP 2004-034645A    Patent document 2: JP 2004-345212A    Patent document 3: JP 2007-210890A    Patent document 4: WO 2004/072053    Patent document 5: WO 2005/078024    Patent document 6: JP 2005-255992A    Patent document 7: JP 2008-195749